dense stand of trees

Feeding on the New Bounty:
Plants as Food

So Many Plants, So Little Food

With the spread of vascular land plants during the Late Silurian and Devonian, greater quantities of potential food became available for animal consumers. There were, however, major obstacles to exploiting this bounty. Algae provide food for a wide range of living marine and freshwater animals, and probably provided the trophic foundation for almost all pre-Devonian ecosystems. But terrestrial plants (and their secondarily aquatic descendants) have a range of adaptations than render them less suitable than algae as food.

In order to support themselves in the absence of standing water, as well as protect themselves from desiccation and ultraviolet radiation, terrestrial plants expanded the use of structural carbohydrates (e.g., cellulose) and synthesized an array of phenolics (aromatic compounds with hydroxyl groups). Structural carbohydrates can be metabolized by a number of microbial and fungal consumers, but are generally indigestible for animals. Lignin, which biochemically distinguishes vascular plants from all other plants, is a complex phenolic polymer that provides strong and chemically resistant, but lightweight structural support; wood is mainly composed of lignins and structural carbohydrates. Cutin, suberin and sporopollenin are three other phenolic polymers that provide resistance to desiccation and physical degradation. These phenolics are resistant to metabolism by most organisms, including bacteria and fungi. Moreover, the biosynthesis of these polymers result in the accumulation of toxic byproducts.

Choosing Herbivory

These biochemical innovations essentially rendered most of the new bounty indigestible and some of it toxic. One way for animals to exploit this resource is to process large quantities of low-quality, indigestible plant matter to extract small amounts of nutritious material. This approach has been adopted by some large-bodied herbivorous tetrapods (e.g., hadrosaur dinosaurs and the larger ungulate mammals). However, the first convincing records of herbivorous tetrapods don’t occur until the Early Permian.

Bulk processing of plant material is not viable for small-bodied invertebrates. In fact, it’s not viable for most small to medium-sized tetrapods as well. Another approach is to selectively feed on the most nutritious—and least toxic—parts. This typically means feeding on new growth and reproductive organs (spores, pollen, seeds and fruits), both of which tend to have less structural (woody) material and lower concentrations of toxic byproducts. This approach has been adopted by a wide range of herbivorous tetrapods, but again they didn’t appear until much later. It has also been adopted by the most diverse and abundant terrestrial herbivores living today: the insects.

However, selective herbivory presents a problem for small-bodied invertebrates. New growth and reproductive organs tend to occur at the ends of branches. They also tend to be ephemeral; new growth becomes old and spores, seeds and fruits are shed. These problems are magnified when the plants are tall. One way to reach these dispersed food patches is through flight, and the reward for reaching patches may have been the primary driving force in evolving insect flight.

Records of winged insects (Pterygota) do not occur until the Middle Carboniferous (Namurian). Questionable or equivocal evidence for wingless insects (e.g., archaeognaths) exist for few late Early and Middle Devonian localities, but evidence for insects in the Late Devonian and Early Carboniferous is limited to suggestive fragments.

Opting for Detritivory

Another way for animals to exploit the expanding vegetation is to feed on dead or discarded plant matter once it has been colonized by bacteria and fungi (i.e., detritus). These decomposers would convert much of the indigestible structural carbohydrates into more readily digestible sugars and deactivate some of the toxins. Moreover, the decomposers themselves could provide nutrition. In fact, detritivory is typically the predominant means through which primary production is utilized in modern terrestrial ecosystems.

Terrestrial plant detritus was probably an important food resource throughout much of the Silurian and the Devonian, but the spread of Archaeopteris forests in the Late Devonian would have substantially increased its importance. This deciduous tree would provide considerable amounts of leaf litter with which to generate detritus. In addition, its canopy would also reduce ultraviolet radiation and moderate the humidity and temperature regimes on the forest floor. All of these effects suggest that detritus, decomposers and detritivores would proliferate.

Fossil evidence of detrivorous invertebrates in the Late Devonian is extremely sparce, but investigations of several Late Silurian (e.g., Ludford Lane), Early Devonian (e.g., Rhynie) and Middle Devonian (e.g., Gilboa) sites suggest that a variety of detritivores were already well established by the Late Devonian. These include millipedes (Diplopoda), arthropleurids (many-segmented arthropods that superficially resemble millipedes), oribatid mites (Acari) and springtails (Collembola). With the exception of the extinct arthropleurids, these arthropods are abundant components in modern leaf litter faunae. Also, with the exception of the arthropleurids, these early detritivores were small (typically < 2 cm).

Predators

A variety of invertebrate predators have also been found in Late Silurian and Devonian deposits. Interestingly, in nearly every locality in which terrestrial invertebrates were collected, predators substantially outnumbered their presumed prey. Part if not most of this disparity can be attributed to a preservational bias in favor of the generally larger-bodied and more extensively sclerotized predators. A possible abundance of unpreserved soft-bodied prey (e.g., oligochaete worms) may also help account for this disparity. Another possibility is that at least some of these predators were amphibious.

Scorpions and trigonotarbids (extinct spider-like arachnids) are the most frequently encountered predators. Centipedes, pseudoscorpions and spiders have also been found.

At least one species of trigonotarbids (Gigantocarinus szatmaryi) has been found at Red Hill as have a new millipede and some unidentified arthropods.

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Scientific Papers:
Algeo, T.J., S.E. Scheckler and J. B. Maynard. 2000. "Effects of the Middle to Late Devonian spread of vascular land plants on weathering regimes, marine biota, and global climate." pp. 213-236. In: P.G. Gensel and D. Edwards (eds.). 2001 Plants Invade the Land: Evolutionary and Environmental Approaches. Columbia Univ. Press: New York.
Beerbower, J.R. 1985. "Early development of continental ecosystems." pp. 47-92. In: B.H. Tiffney (ed.), Geological Factors and the Evolution of Plants. Yale Univ. Press: New Haven.
Beerbower, J.R., J.A. Boy, W.A. DiMichele, R.A. Gastaldo, R. Hook, N. Hotton, III, T.L. Phillips, S.E. Scheckler, and W.A. Shear. 1992. Paleozoic terrestrial ecosystems. pp. 205-235. In: A.K.Behrensmeyer, J.D. Damuth, W.A. DiMichele, R.Potts, H.-D. Sues and S.L. Wing (eds.) Terrestrial Ecosystems throught Time. Univ. Chicago Press, Chicago.

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